Organized travel systems often have multiple different fares for allowing travel from point A to point B. The major airlines have literally thousands of different fares; each with certain restrictions and certain requirements. Moreover, each different airline may charge a different amount for the same trip.
Many internet travel systems store a database with each of the different fares available from each of the different airlines. The lowest fare can be found by searching each of these multiple records. With a high-power computer, the thousands of different fares can be searched relatively efficiently.